An Egyptian policeman shouts at bystanders to keep away from the site of a blast at the Egyptian police headquarters in downtown Cairo, Egypt on Friday.

CAIRO — A truck bomb blasted the main security headquarters in Cairo on Friday, one of a string of four bombings hitting police in the Egyptian capital within a 10-hour period, killing six people. The most significant attack yet in the city fueled a furious backlash against the Muslim Brotherhood amid rising fears of a militant insurgency.

In the hours after the blast, angry residents — some chanting for the “execution” of Brotherhood members — joined police in clashes with the group’s supporters holding their daily street protests against the government. Smoke rose over Cairo from fires, and fighting around the country left 14 more people dead.

The mayhem on the eve of the third anniversary of 2011’s once hopeful revolution pointed to the accelerating, dangerous slide Egypt has taken since last summer’s military ouster of Islamist President Mohammed Morsi: A mounting confrontation between the military-backed government and Islamist opponents amid the escalating militant violence.

Saturday, the anniversary of the start of the 18-day uprising that ousted autocrat Hosni Mubarak, raised the potential for new violence, as both military supporters and the Islamists vowed to take to the streets with rival rallies.

After Friday’s blasts, interim Preisident Adli Mansour vowed to “uproot terrorism,” just as the government crushed a militant insurgency in the 1990s. The state “will not show them pity or mercy,” he said. “We ... will not hesitate to take the necessary measures.”

That could spell an escalation in the crackdown that the government has waged against Morsi’s Muslim Brotherhood ever since his July 3 ouster.

Thousands of Islamists have already been arrested and hundreds killed, with authorities accusing the group of being behind militant violence. The Brotherhood, which allied with some radical groups while in power, denies the claim, saying it is aimed only to justify the drive to eliminate it as a rival. The crackdown has expanded to silence other forms of dissent, with arrests of secular activists critical of the military, security forces and the new administration.

For activists, that has raised deep concerns over a return of a police state despite the government’s promises of democracy.

But among a broad swath of the public, those concerns are eclipsed by fear of the wave of militant bombings and shootings since the coup, which have largely targeted police but increasingly hit in public areas taking civilian casualties. And the public fury has been funneled at the Brotherhood: After Friday’s bombings, TV stations aired telephone calls from viewers pleading with army chief Gen. Abdel-Fattah el-Sissi to definitively crush the group.

“Execution for Morsi and his leaders,” one man shouted through a megaphone to an angry crowd that gathered outside the Cairo security headquarters hit in Friday’s first bombing. A woman held up a picture depicting the Brotherhood as sheep, screaming, “Morsi is the butcher and el-Sissi will slaughter him.”

The day’s violence began with the 6:30 a.m. blast at the capital’s security headquarters, located on downtown Bab el-Khalq Square. Security camera footage that became public showed a white pick-up truck pulling up the building’s gate. A man gets out of it, jumps into another car and drives off. Two policemen inspect the truck for a moment, then return into the headquarters, and two minutes later, it explodes.

The powerful blast ripped down a main avenue that at any other time of day would have been packed with cars and pedestrians — knocking out windows of shops.

The eight-story headquarters’ facade was shattered, with air conditioning units left dangling out of broken windows, and a crater was blasted into the pavement, as deep as a standing man.